This invention relates to the art of nuclear reactor power plants. It has particular relationship to the refueling of the reactors of such plants. In carrying out the refueling, the reactor to be refueled is at the base of a pit in a containment filled with water to a depth of 20 or 30 feet. During refueling, component assemblies of the reactor or from refueling racks are engaged by grippers or grapples of a mast assembly, raised, transported from their position of origin and lowered into the position where they are to be deposited. The component assemblies are highly radioactive and are engaged, raised, transported and lowered under a substantial depth of water. To carry out this operation, there is provided apparatus including a bridge moveable along a track on the containment. A trolley is moveable on a track on the bridge. The trolley carries a mast assembly having a rotatable supporting mast from which the component-assembly engaging-raising-and-lowering means is suspended. This means is sometimes herein referred to as "component-assembly handling mechanism" or "assembly-handling mechanism" or simply "mechanism". With the bridge and trolley at selectably different positions along their tracks, the mast assembly is suspended with the assembly-handling mechanisms at selectably different positions of the area of the pit or of the reactor within the pit.
It is indispensable to successful refueling that the mechanism on the mast assembly be positioned during each operation to engage and raise the exact assembly which is selected for transport. Since the reactor is under water, the positioning of the mechanisms must be carried out with the necessary precision with at best a heavily clouded view of the nuclear core. It is also necessary that the depth of the mechanisms in the water be known. The tracks on the bridge and containment define a coordinate system whose coordinates serve to determine the positions of the mechanisms on the mast assembly. The tracks in Swidwa, and predominantly in prior art apparatus, are linear and at right angles to each other defining a Cartesian coordinate system. With the tracks of other configurations, other coordinate systems may define the positions of these mechanisms. For example, if the bridge moves in a circular track on the containment and the trolley moves on a radial track, the coordinate system would be a polar coordinate system.
The coordinates of the positions of the bridge and trolley along the tracks are magnitudes which indicate the positions of the engaging-raising-and-lowering means over the pit. In addition, there are provided indications of the vertical positions of these mechanisms. The mechanisms are raised or lowered by a hoist which is typically a winch.
In accordance with the teachings of the prior art the positions of the mast assembly along the tracks are derived from selsyn-type indicators. The readings of the indicators for different positions of the mechanisms of the mast assembly are compared with marks along the containment walls. The marks are coordinated with the indications of the selsyns by a calibration process. Impairment of the calibration was experienced and when this occurred, recalibration was a difficult and time consuming experience. The height of the mechanisms on the mast assembly was determined from marks on a tape along the assembly. Recalibration in this case was also difficult and time consuming.
In accordance with the teachings of the prior art, index marks were also provided on the trolley and bridge to locate the component assemblies to be moved in the reactor core. The index marks served to position the mast assembly over the theoretical location of a component assembly. If the assembly is out of position, difficult visual determinations were required to make the necessary adjustments. This operation was time consuming and required the utmost caution to preclude damage to the component assemblies.
It is the object of this invention to overcome the difficulties and drawbacks of the prior art. An object of this invention is to provide refueling apparatus for a reactor having facilities, whose calibration shall be maintained throughout a refueling operation, for reliably and precisely identifying the position of the mast assembly with reference to the reactor core. A further object of this invention is to provide refueling apparatus including position-visual facilities for aligning the mast assembly with the component assembly to be transported for identifying and recording the location of component assemblies to be transferred and, if desirable, to provide a permanent record of the refueling and core-mapping operation.